In order to cope with an environmental problem of global warming, biomass fuel has been notified which does not influence on the increase/decrease of carbon dioxide in the atmosphere in a view point of fuel life cycle, that is, which absorbs carbon oxide due to the photosynthesis in the growing process of plant by an amount almost same as carbon dioxide generated at the time of burning. Among the biomass fuel, woody pellet is in general a solid fuel which is formed by grinding and subjecting a compression forming with respect to scrap wood or remaining wood in the forest etc. generated by final cutting or forest thinning. The woody pellet is stable in its shape and quality such as a water content ratio. Thus, since the woody pellet can be treated easily as compared with other biomass fuel, the development of the use of the woody pellet has been examined widely as fuel for various kinds of heat sources such as a hot-water boiler, a steam boiler, a heat source of a greenhouse, a water heater, a heating equipment, a regenerator of absorption refrigeration cycle.
Since the woody pellet generates a relatively large amount of combustion ash, the discharging of the combustion ash is a problem for performing a stable burning. For example, a patent document 1 proposes a burning apparatus in which ceramic particles are covered all over a porous plate having many air ejection holes to form a fire bed, then woody pellets are dispersed on the fire bed of the ceramic particle layers, and combustion air is flowed into the ceramic particle layers from the lower side of the porous plate to thereby flow and burn the woody pellets. According to this apparatus, the combustion ash is scattered into a secondary combustion chamber on the downstream side together with the combustion air and the combustion gas flowing through the ceramic particle layers, then moves downward and deposits within the secondary combustion chamber and is suitably discharged to the outside of the combustion chamber. On the other hand, the ash not deposited in the secondary combustion chamber is flown together with the combustion gas and collected by a cyclone etc.